a) Field of the invention:
The present invention relates to an imaging optical system of a camera with an illuminator, capable of photographing (macro-photographing) an object located at a distance closer than at a minimum focusing distance (namely, in a macrophotographic region).
b) Description of the prior art:
In recent years, an illuminator housing type camera having an automatic focusing function has been popularized, and consumers have demanded that such a camera should have relatively high accuracy, compact size and low cost. For this purpose, the adjustment of the amount of light in the case where an illuminator is employed is made by rendering the amount of radiation of the illuminator constant to control the aperture of a stop. Further, in most cases, the stop is also used as a shutter.
Where such a camera is used for macrophotography, it needs to make use of a stroboscope as the illuminator in order to assure portability and needs to prevent a blur in photographing. Further, it is necessary to render the aperture of the stop extremely small as compared with the case of ordinary photography for the adjustment of the amount of light. In addition, a lens shifting distance for focusing largely increases as compared with that in an ordinary photographic region, so that the camera has defects that the design of a lens shifting mechanism is considerably difficult and, as a result, oversizing and rises in manufacturing costs are caused.
Although measures may be taken to hold the lens shifting distance constant for focusing to some extent and the stop may be stopped down with respect to an object point to increase a depth of field, which thereby brings about a falsely in-focus condition, there is the defect that a good focus is not obtained in comparison with the case of the foregoing.
Accordingly, another type of camera is proposed in which a variable focal length lens making use of such an optical element that a refracting power is changed by pressure, voltages and currents is adopted as a photographic lens and in which the lens shifting distance for focusing is diminished so that the in-focus can be brought about. The lens such that the refracting power is changed by pressure is proposed by Japanese Patent Preliminary Publication Nos. Sho 62-124535 and Sho 63-208817 and one such that the refracting power is changed by voltages and the like is proposed by Japanese Patent Preliminary Publication Nos. Sho 62-153933, Sho 62-91914, Sho 61-140908 and Sho 61-160714.
The lenses composed of such optical elements, however, have disadvantages that in general, it is difficult to manufacture a lens with a large diameter and the scattering and absorption of light by the material constituting the variable focal length lens is increased compared with scattering by optical glass, and consequently a fear is aroused that the performance of the camera in ordinary photography will be lost.
Moreover, the use of the lens of the type which varies in configuration renders it difficult to perform accurate control of the pressure applied to the lens for deformation, and if the accurate control is intended to be realized, a driving device and control circuit will be large-sized, with the result that the raise of manufacturing costs is brought about. Further, of variable focal length lenses of the type in which the refracting power is changed by voltages and the like, one which is a liquid crystal lens and has a large diameter in particular causes the reduction of an operating speed and the deterioration of transmittance due to the increase of its thickness. As such, this lens is used as a Fresnel lens in most cases, but the Fresnel lens has the drawback that imaging performance in ordinary photography is reduced.
Furthermore, in the use of the liquid crystal lens, if it is intended that the optical system is constructed from a single liquid crystal lens, it will inevitably need polarizing plates, by which the amount of transmitting light is reduced to less than half, so that the loss of the amount of light in the ordinary photography is enormous. Although Japanese Patent Publication No. Sho 61-45812 offers the optical system in which two liquid crystal lenses whose orientation directions intersect at right angles are combined with each other so that no polarizing plate is used, this structure possesses the defect that astigmatism is generated in the ordinary photographic region.
On the other hand, the stop of the camera of the type, that is used in common with the shutter, renders accurate control of its aperture difficult and the aperture reduced to less than some extent constitutes a danger of causing unexposure.
Consequently, the following improved means are suggested.
A first means is such that the lens is shifted in a moderate position for fixation, the aperture of the stop is fixed to such size as to exhibit a proper exposure at a a certain distance, the depth of field is made large until the focus is taken, and the exposure depends on the latitude of a film, which is proposed by Japanese Patent Preliminary Publication Nos. Sho 58-152227, Sho 59-149334, Sho 56-151919, and Sho 59-201027 and Japanese Utility Model Preliminary Publication No. Sho 61-88122.
A second means is such that in addition to the stop for ordinary photography, another stop for close-up photography smaller in aperture is together provided and a small aperture stop stable in the close-up photography is available, which is also proposed by Sho 59-201027 mentioned above.
Finally, as a third means, a stop utilizing the physics of an aperture stop as a liquid crystal stop (which will be hereinafter referred to as a physical stop) is widely known which is constructed from a member capable of changing the transmittance electrically or magnetically to control accurately the aperture of the stop, and, for example, stops making use of polarization (Japanese Utility Model Preliminary Publication No. Sho 54-154741) and making no use of the polarization (Japanese Patent Preliminary Publication No. Sho 62-105125) are available.
In the first means, however, a mechanical stop also serving as the shutter is used in most cases and its accuracy is low due to high-speed operation. Moreover, it is inevitable that in order to prevent the unexposure, the aperture of the stop is made large and the photography in a macrophotographic region is set to over-exposure so that a print by almost proper exposure is secured depending on the latitude of the film as stated above or the macrophotographic region is set at some distance. Either case is unfavorable for photographic accuracy per se of the camera.
Further, the second means has disadvantages that since two stops are disposed together, the mechanism is bulky and the camera is large-sized.
The third means has a serious defect that since the scattering and absorption of light of material constituting the physical stop increases as compared with scattering by optical glass, the performance of the image in ordinary photography will deteriorate. Further, most of the physical stops, having poor light-blocking properties, are such that when an aperture portion is completely larger than a light-blocking portion, the effect of the stop is brought about, whereas when the stop is stopped down and the area of the light-blocking portion becomes larger, the amount of light transmitted by the light-blocking portion increases, so that a stopping-down effect of making the depth of field large will not be secured and if the amount of the transmitted light becomes greater than that of light traversing the aperture portion, F-number will become close to an open value of the aperture.
By the way, since such an illuminator that the amount of radiation can be adjusted in response to a photographing distance is difficult to be manufactured as a small-sized one and comes expensive, it cannot be adopted.